As is well known, for example, automobile exterior parts or exterior design parts such as decorative parts including, for example, emblems, front grills (radiator grills), and door handles of automobiles are subjected to decorative chrome plating for purposes of improving aesthetic appearance, increasing surface hardness to prevent scratch, and furthermore providing corrosion resistance to prevent rust.
More specifically, in a decorative chrome-plated part having a body made of metal or a resin material such as ABS, the body is sequentially subjected to copper plating, non-sulfur nickel plating, bright nickel plating, and corrosion distribution nickel plating as surface preparation for chrome plating, and then chrome plating is performed for the corrosion distribution nickel plating layer by a hexavalent or trivalent chromium plating bath. On the hexavalent chrome plating layer, a passive film is formed by a wet oxidation treatment such as an anodic electrolytic oxidation, thus obtaining a composite film layer structure (Patent Citation 1). These are intended for a multilayer structure which prevents corrosion for an increase in corrosion resistance and are described as follows.
In other words, the chrome plating layer in the surface constitutes a composite structure together with the underlying nickel plating layer, and the nickel plating layer constitutes a composite structure together with the non-sulfur nickel plating layer, bright nickel plating layer, and corrosion distribution nickel plating layer to distribute corrosion current for an increase in corrosion resistance. Furthermore, the corrosion distribution nickel plating is microporous nickel plating or microcrack nickel plating which generates microcracks by high stress. By action of these types of corrosion distribution nickel plating, the chrome plating layer in the surface includes fine pores (microporous) or fine cracks (microcracks). A number of the micropores or microcracks cause corrosion current to be distributed, thus preventing local corrosion of the underlying bright nickel plating layer. This results in an increase in corrosion resistance.
The total thickness of all of the plating layers of the aforementioned composite film layer structure except the chrome plating layer in the surface is about 5 to 100 micrometers, and the top-most chrome plating layer necessary for keeping the aesthetic appearance is resistant to corrosion. Accordingly, the composite film layer structure can give a decorative chrome-plated part with a design exploiting the advantage of white silver color of the chrome plating layer over long periods.
Moreover, the long employed hexavalent chromium plating is excellent in white metal bright appearance. However, hexavalent chrome is being subject to strict environmental restrictions in recent years, and Non Patent Citation 1 discloses as a decorative trivalent chromium plating technique replaced for the hexavalent chromium plating, TriChrome Plus process, TriChrome Light process, and TriChrome Smoke process using a single cell-type trivalent bath and in addition an envirochrome process and a twilight process using a double cell-type trivalent bath.
[Patent Citation 1]
    Japanese Patent Laid-open No. 2005-232529 Publication[Non Patent Citation 1]    “Surface Technology”, the Surface Finishing Society of Japan, Vol. 56, No. 6, 2005, P 20-24